This invention pertains to a method and apparatus for depositing metal alloy films. More particularly the invention pertains to a method and apparatus for electroplating alloy metal films.
In the semiconductor art alloys such as Aluminum-Copper are used to produce interconnect metallizations.
For various composition material films it is important to strictly control the proportions of constituent materials in the compound. In the case of solder alloys, for example, the proportions of constituent metals effect critical properties including melting point temperature, and adhesion.
One conventional method for composition film deposition is sputtering. In sputtering, a source material target having a predetermined composition is arranged in a vacuum chamber opposite a substrate onto which a film having a composition corresponding to the predetermined composition is to be deposited. The chamber is charged with a small amount of reactive or inert gas. A high voltage DC and/or high frequency discharge is struck in the gas to cause ions formed from the gas to collide with the target and knock off particles which are deposited on the substrate. Unfortunately, sputtering requires vacuum processing, complex power supplies, and is generally only suited to single substrate processing. While sputtering machines can be scaled up to handle multiple substrates, although there is little gained in economies of scale by doing so.
Another method for depositing alloy films is electroplating. In one conventional method, electroplating is carried out using inert electrodes, and an electrolyte solution bearing a plurality of solutes compounds including metals to be deposited. However, in electroplating with inert electrodes, solutes need to be constantly replenished in order to sustain continued plating. Moreover, in the case of alloy plating, solutes corresponding to different constituents of the alloy may become depleted from the electrolyte solution at different rates and this leads to uncontrolled variations in the alloy composition obtained.
In another conventional method, electroplating is carried out using an anode made of an alloy, corresponding to the alloy film which is desired. However, in practice, it has been found that the proportions of constituents in the alloy film do not, in fact, correspond to the constituents of the alloy anode. Moreover, during the course of plating, the electrolyte tends to shift towards one of the metals of the alloy in an uncontrolled manner.
An additional disadvantage of using an alloy anode is that alloy anodes tend to require more power to sustain a given rate of dissolution, compared to pure metal electrodes made of any of the alloy constituent metals.
It is also possible to produce an alloy electroplated film by using electrolyte comprising compounds (e.g., salts) of different metals which are included in the alloy, and a pure metal anode. However, as the electrolyte is used during plating, its composition shifts toward the metal of the pure metal anode.
In addition to the desirability of depositing homogeneous alloy films, it is desirable, for certain applications such as high corrosion resistance films, and thin film microelectronic devices, to produce Compositionally Modulated Multilayers (CMM), which are also know as Functionally Graded Materials (FGM). CMM or FGM films, are characterized by constituent element proportions which vary in a predetermined manner as a function of depth in the film.
One conventional method for obtaining multilayer metal films is to repeatedly transfer a substrate between a plurality of plating baths each of which deposits a film of a particular composition on the substrate or an underlying film. However, using multiple plating baths is complicated, and such a process is also hampered by the problem of contamination and oxidation during transfer of the substrate which can passivate a surface layer on the substrate and create difficulty for further plating. The oxidation might create another layer between the two desired layers rendering the substrate unsuitable for its application
It would be desirable to have a method and apparatus for depositing CMM films that avoids interfacial contamination and oxidation between layers of the CMM.
It would also be desirable to have a system and method which overcomes the above mentioned shortcomings.
Briefly, one embodiment of the present invention provides, an apparatus for depositing a composition film comprising a first metal and a second metal on a substrate. The apparatus includes a container for containing an electrolyte including a first dissolved disassociated compound including the first metal, and a second dissolved disassociated compound including the second metal. Additionally there is a support for an first anode which includes the first metal, a support for a second anode which includes the second metal, and a servo for changing a contact area between the first anode and the electrolyte.